It's a four-bedroom apartment for one childless couple, so this particular case doesn't support the idea that there was less demand. And the population of London is within 20% of what it was.
The less demand is that the maid lives in the place they work, so do not need their own property to live in. The article seemed to suggest that having live-in staff was common compared to car ownership etc, so I would imagine there were a lot of adults living in their places of work, thus not needing to rent their own apartments etc so reducing demand.
